connect() problem

[harmodrew]

Hello you all!
I'm a newbie in Qt programming so my question will probably seem a stupid question but all guides I read didn't help me...
I'm developing a C/C++ program in Win Vista & Qt Creator IDE with the Qt framework. It is a Qt GUI Application (the one you can create by clicking New -> project)
I'd like to connect the event of clicking a PushButton with a function I wrote earlier. Here is the code of my mainwindow.cpp file:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QPushButton>
#include<QMessageBox>
class act: public QObject
{
 public slots:
     void buttonClicked()
     {
         QMessageBox("Title","Msg",QMessageBox::NoIcon,QMessageBox::Ok,QMessageBox::NoButton,QMessageBox::NoButton);
     }
 };
  MainWindow::MainWindow(QWidget *parent) :
     QMainWindow(parent),
     ui(new Ui::MainWindow)
 {
QPushButton *PB= new QPushButton(this);
     PB->setText("Button text");
      act a;
     connect(PB, SIGNAL(clicked()), &a,SLOT(buttonClicked()));
     PB->show();
     }
  MainWindow::~MainWindow()
 {
     delete ui;
 }

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QPushButton>
#include<QMessageBox>
class act: public QObject
{
 public slots:
     void buttonClicked()
     {
         QMessageBox("Title","Msg",QMessageBox::NoIcon,QMessageBox::Ok,QMessageBox::NoButton,QMessageBox::NoButton);
     }
 };
  MainWindow::MainWindow(QWidget *parent) :
     QMainWindow(parent),
     ui(new Ui::MainWindow)
 {
QPushButton *PB= new QPushButton(this);
     PB->setText("Button text");
      act a;
     connect(PB, SIGNAL(clicked()), &a,SLOT(buttonClicked()));
     PB->show();
     }
  MainWindow::~MainWindow()
 {
     delete ui;
 }

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It is executed correctly but when I push the button the message doesn't show!

What could be the problem?

Thanks a lot, harmodrew

[squidge]

In your constructor you create a local variable which is destroyed when the constructor has finished.

Why not use the reference you already have?

Secondly, the class declaration should be in a seperate header file to the definition so it can be easily compiled by the meta object compiler.

[harmodrew]

Why not use the reference you already have?

What reference?

[leoalvesmachado]

To use Message Box, use the following example, for instance

QMessageBox msgBox;
 msgBox.setText("The document has been modified.");
 msgBox.exec();

QMessageBox msgBox;
 msgBox.setText("The document has been modified.");
 msgBox.exec();

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[harmodrew]

searching around the Net I've found that all signal/slot classes must include in the private section the macro Q_OBJECT.
Now the problem is that if I include this macro I have the error:
"collect2: Id returned 1 exit status"
and clicking the message of the error it doesn't bring me to the row affected...

[squidge]

What reference?

act a;
connect(PB, SIGNAL(clicked()), &a,SLOT(buttonClicked()));

act a;
connect(PB, SIGNAL(clicked()), &a,SLOT(buttonClicked()));

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connect(PB, SIGNAL(clicked()), this,SLOT(buttonClicked()));

connect(PB, SIGNAL(clicked()), this,SLOT(buttonClicked()));

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Secondly, when you are finished, choose "run qmake" in QtCreator before trying to run. This will generate the rules to run MOC.

[harmodrew]

So, here are my project files (I've split the class "actionObj" from the mainwindow file).

mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QPushButton>
#include "actions.h"
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    QPushButton *NewButton= new QPushButton(this);
    NewButton->setText("Button");
 
    actionsObj *ao;
    ao = new actionsObj;
    connect(NewButton, SIGNAL(clicked()), this,SLOT(ButtonClicked()));
    NewButton->setGeometry(50, 40, 300, 200);
    NewButton->show();
 //  ui->setupUi(this); if I uncomment this, the button isn't clickable!
 
}
 
MainWindow::~MainWindow()
{
    delete ui;
}

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QPushButton>
#include "actions.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    QPushButton *NewButton= new QPushButton(this);
    NewButton->setText("Button");

    actionsObj *ao;
    ao = new actionsObj;
    connect(NewButton, SIGNAL(clicked()), this,SLOT(ButtonClicked()));
    NewButton->setGeometry(50, 40, 300, 200);
    NewButton->show();
 //  ui->setupUi(this); if I uncomment this, the button isn't clickable!

}

MainWindow::~MainWindow()
{
    delete ui;
}

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actions.cpp:

#include "actions.h"
#include <QMessageBox>
 
void actionsObj ::ButtonClicked()
{
    QMessageBox msg;
    msg.setText("Hello. You've clicked the button");
    msg.exec();
}

#include "actions.h"
#include <QMessageBox>

void actionsObj ::ButtonClicked()
{
    QMessageBox msg;
    msg.setText("Hello. You've clicked the button");
    msg.exec();
}

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actions.h:

#ifndef ACTIONS_H
#define ACTIONS_H
#include <qobject.h>
 
class actionsObj : public QObject
{
private:
    Q_OBJECT
public slots:
    void ButtonClicked();
};
 
#endif // ACTIONS_H

#ifndef ACTIONS_H
#define ACTIONS_H
#include <qobject.h>

class actionsObj : public QObject
{
private:
    Q_OBJECT
public slots:
    void ButtonClicked();
};

#endif // ACTIONS_H

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mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
 
namespace Ui {
    class MainWindow;
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
 
private:
    Ui::MainWindow *ui;
};
 
#endif // MAINWINDOW_H

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
    class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

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main.cpp:

#include <QtGui/QApplication>
#include "mainwindow.h"
#include <QAction>
#include <QString>
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QString title="MainWindow";
 
    MainWindow w;
    w.setGeometry(100, 100, 500, 500);
    w.show();
    return a.exec();
}

#include <QtGui/QApplication>
#include "mainwindow.h"
#include <QAction>
#include <QString>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QString title="MainWindow";

    MainWindow w;
    w.setGeometry(100, 100, 500, 500);
    w.show();
    return a.exec();
}

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But now I have the following error:

collect2: Id returned 1 exit status

and the IDE doesn't bring me to the line affected...what could be the problem? it's possible that a simple program like this, even if I'm a starter, is so complicated and full of problems??

Thanks a lot

[Lykurg]

connect(NewButton, SIGNAL(clicked()), this,SLOT(ButtonClicked()));

connect(NewButton, SIGNAL(clicked()), this,SLOT(ButtonClicked()));

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.
"this" is wrong because it means your main window, but your slot is in your actionsObj. Also you are producing memory leaks! (ao = new actionsObj

[harmodrew]

I've changed the code and now it works!

QPushButton *NewButton= new QPushButton(this);
NewButton->setText("Button");
 
actionsObj *ao= new actionsObj;
connect(NewButton, SIGNAL(clicked()),ao,  SLOT(ButtonClicked()));
NewButton->setGeometry(50, 40, 300, 200);
NewButton->show();
//ui->setupUi(this);

QPushButton *NewButton= new QPushButton(this);
NewButton->setText("Button");

actionsObj *ao= new actionsObj;
connect(NewButton, SIGNAL(clicked()),ao,  SLOT(ButtonClicked()));
NewButton->setGeometry(50, 40, 300, 200);
NewButton->show();
//ui->setupUi(this);

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the only problem now is that if I uncomment the last row the button isn't clickable anymore. What could be the problem?

[harmodrew]

ok, I've done it!
simply, the ui->setupUi(this); row is to be written before the other rows, not in the bottom
thanks you all

[harmodrew]

here I am again for the same problem...
mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
 #include <QObject>
 
class act :public QObject
{
    Q_OBJECT
 
};
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
 
    act *ao= new act;
}
 
MainWindow::~MainWindow()
{
    delete ui;
}

#include "mainwindow.h"
#include "ui_mainwindow.h"
 #include <QObject>

class act :public QObject
{
    Q_OBJECT

};

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    act *ao= new act;
}

MainWindow::~MainWindow()
{
    delete ui;
}

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this little code returns me the same error:

collect2: Id returned 1 exit status

what it refers to??
in addition if I comment the Q_OBJECT line it compiles without errors!

[Lykurg]

put your class act in a single file or make sure moc runing right for your class. Simpliest is every class a single file!

[harmodrew]

thanks for the reply, it helped! but, why are we obliged to write the class in an another file to make it work? what does it change?

[squidge]

Its to do with the meta object compiler. You can have classes in the same file as your .cpp if you want, but if they include the Q_OBJECT macro, then you need to run MOC, so ideally those classes should be in .h files instead.

[harmodrew]

oh, it's clear now! thank again!